Sand dunes occur throughout the world, usually in coastal and/or desert environments. Sand is made up of particles that are light enough to be moved by the wind but too heavy to remain suspended in the air. Particles finer than sand are usually considered as dust and particles heavier than sand are usually considered as pebbles or gravel. Dune formations often occur in areas with the right topography, an abundant supply of sand (usually with little vegetation), and sufficient wind. It is the interaction between these three essential components that results in dune formation, but the grain sizes of the sand can also influence the characteristics of resulting dunes. In addition to natural beauty of the dunes themselves, these unique environments also host a variety of plants and animals adapted to life in a world of drifting sand.